1. Field of the Invention
The present invention relates to an automobile exhaust gas recirculating system for feeding a portion of automobile exhaust gases back into a combustion chamber for reuse in combustion with a fresh combustible air-fuel mixture.
2. Description of the Prior Art
The automobile exhaust gas recirculating (EGR) system employed in commercially available automobiles and designed, although not exclusively, to minimize the NOx emission is available in two types: an external EGR system in which an automobile exhaust system is connected with an automobile fuel intake system for feeding a controlled amount of the exhaust gases discharged from the combustion chamber back to the combustion chamber through the fuel intake system, and an internal EGR system in which a controlled amount of the exhaust gases being charged to the exhaust system is allowed to substantially remain within the combustion chamber or to back-flow into a portion of the intake system adjacent the combustion chamber.
As compared with the external EGR system, the internal EGR system has an advantage in that, since that portion of the exhaust gases, the product of combustion of a combustible air-fuel mixture which has taken place in the combustion chamber, is allowed to substantially remain within the combustion chamber or to back-flow into a portion of the intake system adjacent the combustion chamber, the temperature of that portion of the exhaust gases is still so high as to act as a "heater" for heating a fresh combustible mixture supplied into the combustion chamber during the suction stroke of the next succeeding cycle of operation of the engine, thereby facilitating atomization of the fresh combustible mixture. Therefore, for a given amount of the exhaust gases utilized in the same combustion chamber in which such exhaust gases have been produced, the combustibility can be stabilized more effectively by the internal EGR system than by the external EGR system.
One way to accomplish the internal exhaust gas recirculation which is well known in the art is to provide an intake-exhaust overlap period, that is, a period during which an intake valve for selectively opening and closing an intake port and an exhaust valve operable in a manner opposite to the manner in which the intake valve operates for selectively closing and opening an exhaust port are simultaneously opened. During the intake-exhaust overlap period, that is, the simulataneous opening of the intake and exhaust valve, the exhaust gases being discharged to the exhaust system as a result of a progressive opening of the exhaust valve are partially drawn into the combustion chamber by the effect of the negative pressure which has been developed inside the intake system during the opening of the intake valve. Depending on the length of the intake-exhaust overlap period and/or the magnitude of the negative pressure developed inside the intake system, a portion of the exhaust gases may back-flow into the intake system, particularly into a portion of the intake system adjacent the combustion chamber. That portion of the exhaust gases substantially remaining within the combustion chamber is mixed with the subsequently supplied combustible mixture for the purpose discussed above.
An example of the prior art internal EGR system is disclosed in, for example, the Japanese Laid-open Patent Publication No. 55-112814, published in 1980; and the Japanese Laid-open Utility Model Publication No. 55-180906, published in 1980.
Apart from the disclosure made in any one of the above-mentioned prior art publications, where both the valve timing relationship between the intake and exhaust valves and the intake-exhaust overlap period are fixed, a problem has arisen in that, since the amount of that portion of the exhaust gases drawn into the combustion chamber depends on the negative pressure developed in the intake system, not only can that portion of the exhaust gases not be controlled to a value appropriate for a particular engine operating condition, but also a loss of temperature of that portion of the exhaust gases tends to occur because that portion of the exhaust gases has been discharged to the exhaust passage and the temperature thereof has been consequently lowered due to the contact thereof with the wall defining the exhaust passage.
In an attempt to substantially eliminate the problems inherent in the system wherein the intake-exhaust overlap period is fixed, both Japanese Laid-open Patent Publication No. 55-112814 and Japanese Laid-open Utility Model Publication No. 55-180906 disclose systems in which the valve timing mechanism is designed to vary the intake-exhaust overlap period. Although the use of a variable intake-exhaust overlap period is advantageous in that pumping loss can be minimized, the valve timing mechanism tends to have a relatively complicated structure and, therefore, is expensive to manufacture.